The present invention is generally related to a method for electrically interconnecting a first plurality of contact elements in an array of elements on a first electrical article to a second plurality of contact elements in an array of elements on a second electrical article. The invention specifically relates to electrically interconnecting contact elements on a flexible film circuit or on a rigid printed circuit board with a flat, flexible cable or flexible film circuit and is further directed to the resulting apparatus and to methods of manufacture.
Flexible printed circuit boards and flexible cables constructed from polymeric film, (e.g.) polyester or polyimide film, substrates are generally known in the art. Such films are lightweight, flexible and thin. Varying circuit shapes, dimensions, circuit arrangements and different length cable arrangements may be constructed using polyester. However, a disadvantage to the use of polyester is its relatively low tolerance to heat. Lap joints have been formed using a hot bar to cause solder reflow on polyimide substrates, but using a similar process with a polyester substrate could burn or melt polyester. Unfortunately, polyimides are substantially more expensive than polyesters.
When electrical connection was desired between printed circuit boards, cables were often used having a sufficient number of wire leads to connect the bond pads on one circuit to another circuit. Such electrical connections were most commonly made by inserting the cable structure into a mechanical plug-in formed on the edge of one circuit. Such plug-ins provided excellent electrical and mechanical bonding. However, they were expensive to make and difficult to install during assembly. The plug-in type edge bonding connectors replaced complex point-to-point connections using discrete copper wires.
Flexible circuit boards have been joined electrically (and physically) either by sing anisotropic adhesives (which are electrically conductive in the vertical or z-direction, but are substantially electrically insulating in the horizontal x, y, or x-y plane) or by physically crimping the boards together. Unfortunately, adhesives require a relatively large surface area to ensure adequate conductivity. In addition, mechanical crimps can be prone to failure and have limited conductivity.
Conventional circuit board assemblies and methods of their construction do not appear to provide an economical and reliable solution to interconnection and construction challenges. Therefore, a printed circuit board assembly comprising a flexible circuit member and method for connecting a printed circuit board to such a flexible member that addresses the above-identified deficiencies is a demonstrable goal in this art.
The invention involves a new process for electrically and mechanically joining arrays of conductors on rigid or flexible printed circuits with corresponding conductors on a second flexible circuit board or to a collated flat, flexible cable (FFC). Corresponding bonding areas are overlapped. In the bonding area mated pads or conductor surfaces with a solder layer are contacted. The overlapped bonding areas are inductively heated under pressure to mechanically and electrically bond the areas. Adhesive surrounding the conductors is thermally activated to bond the circuit boards together.
In the process a flexible member comprising an array of flat copper conductors formed on a flexible film comprising a dielectric sheet is electroplated with tin-lead solder. Surface insulation or solder masks are locally omitted or removed from conductor surfaces. The conductors on the circuits or cables are overlapped onto the corresponding conductors and the solder plating is fused by inductively heating the copper to join the two conductor arrays in a discrete bonding area. Adhesive on the dielectric film between conductors is thermally activated to bond the film on the two cable arrays together. This insulates the newly formed electrical connections and seals them in a bonding area from attack by moisture and chemical pollutants in the production or use environment.
There are several advantages over present technology. The solder and adhesive can act alone to electrically and mechanically connect the members. The conductor arrays on flexible circuits and on FFCs can be joined without adding additional materials for electrically and mechanically connecting, insulating and sealing the connections. The process of the invention can be used on flex circuits and cables having low temperature substrate or insulation (PET polyester film, for example), unlike conventional soldering methods which require heat resistant insulation or substrate (like polyimide film, for example). In addition, this process can be used in place of conventional metal-plastic plug-like connector devices to give joinable connections for electronic modules which are interconnected by flex circuits and FFCs. The inductively heated joint according to this invention does not limit the current carrying capability of the interconnect beyond that of the current capability of the conductors, whereas conventional mechanical crimps may limit the current carrying capability at the interconnect.
The invention is directed to a preferred assembly of electrically connected printed circuit boards which comprises a first flexible printed circuit board including a substrate and an electrically conductive layer comprising a plurality of conductive leads or traces bonded to the substrate pads, and having a first plurality of solderxe2x80x94covered electrically conductive leads or traces disposed on the substrate with adhesive present between the pads or leads; a second flexible printed circuit board including a substrate and an electrically conductive layer bonded to the substrate with insulating adhesive, and having a second plurality of solder covered electrically conductive leads disposed on the substrate with adhesive present between the leads; and wherein the first plurality of leads are mechanically and electrically connected to the second plurality of leads. The adhesive between the first plurality of leads is bonded to the adhesive between the second plurality of leads, thereby insulating the electrically connected leads one from another.
The invention is further directed to a method for connecting a first printed circuit board to a second printed circuit board, each of the boards having corresponding or mated, solder covered copper leads or connector pads disposed on the circuit board and adhesive disposed between the leads or pads. The method comprises the steps of contacting the leads of the first board with mated leads of the second board; inductively soldering the leads of the first board to the leads of the second board to establish an electrical connection; and bonding the adhesive disposed between the leads of the first board to the adhesive disposed between the leads of the second board, thereby sealing adjacent pairs of mated leads one from another.